This invention relates to an air gun firing sensor apparatus and system for monitoring the firing of an air gun used for the generation of seismic energy in a medium for determining the actual instant of firing of the air gun.
In seismic surveying, air guns are utilized to generate powerful seismic energy impulses in order to investigate subsurface geological conditions and formations in the earth. For this purpose, one or more such air guns are each charged with pressurized gas, usually compressed air, which is temporarily stored in them. At the desired "shot" instant, the air guns are fired, and the pressurized gas is abruptly released into the surrounding medium. In this manner, powerful seismic energy impulses are generated capable of penetrating deeply into subsurface material to be reflected and refracted by the various geologic strata and formations. The reflected and refracted impulses are sensed and recorded to provide information and data about the geological conditions and formations.
For many applications, in both marine and land seismic exploration, it has been found advantageous to generate a plurality of seismic energy impulses from an array of air guns in order to provide a composite seismic signal of predetermined amplitude and frequency content and in spacial or array arrangements. For marine seismic surveying an array of air guns having various firing chamber volumes may be utilized in order to produce a composite seismic signal having a predetermined frequency distribution as air guns of different firing chamber volumes generate seismic energy impulses with different frequency spectra. In such applications, as well as those in which air guns are repetitively fired or fired in sequence, it is desired to control the timing of the solenoid control valves which are used to trigger the firing of the respective air guns.
For further background on air guns and solenoid valves reference may be made to U.S. Pat. Nos. 3,249,177; 3,379,273; 3,779,335; 3,588,039; 3,653,460; 3,997,021 and 4,038,630.
In the past, attempts have been made to synchronize the timing of the solenoid-controlled valves which trigger the individual air guns. However, the time at which a solenoid plunger actually becomes actuated in response to an electrical signal will vary with each valve, and then there are further variables caused by the mechanical structure of each air gun, such as dimensional tolerance variations, wear of parts, slight differences in shuttle mass, different frictional drag of moving parts, and so on. When the individual air guns are not actually operating in accordance with the prescheduled precisely timed relationship, then the downgoing seismic waves are not in accordance with the predetermined survey program, and less than optimum results are obtained. For example, if the survey program calls for all of the air guns to fire in synchronism, and they are not synchronized, the downgoing seismic waves are misaligned. Misalignment may cause reduction in total amplitude of the downgoing seismic waves or cause the generation of seismic waves with undesirable frequency spectra, or both, resulting in data collection with reduced penetration and resolution.
A solution to the problem of inaccurate firing ("shot") instant as set forth in U.S. Pat. Nos. 4,034,827 and 4,047,591 assigned to Texas Instruments, Incorporated has been to sense movement of a gas-releasing shuttle within each air gun by magnetic, electromagnetic or electrostatic field effects to establish the precise instant of firing of each individual air gun. The firing signals applied to the respective air gun solenoid valves are then delayed or advanced relative to each other in subsequent firings to properly synchronize or sequence the firings of the respective air guns. In those patents, the air guns are modified in order that a field such as a magnetic field be set up. The air guns are further modified in order that variations in that field caused by movement of the shuttle can be detected.
In the final paragraph of each of the above patents, possible arrangements for producing a magnetic, electromagnetic or electrostatic field and sensing variations in that field were listed. A primary disadvantage of each of the embodiments suggested in these patents is that one or more complex modifications of the air gun itself is required. Such modification of an air gun to include means for generating a magnetic, electromagnetic or electrostatic field plus means to sense variations in that field are generally complicated procedures which cannot be accomplished by an operating crew, and even such modification of air guns yet to be manufactured is costly. Further, any repairs by an operating crew to a sensor which is built into the air gun is time-consuming and costly.
In copending applications Ser. Nos. 917,792 (issued as U.S. Pat. No. 4,210,222 on July 1, 1980) and 917,794, both filed on June 22, 1978 and assigned to the same assignee as the present application there are described how and why a pressure transducer can be used advantageously to monitor the firing of individual air guns being used in an array of air guns for seismic surveying. The application Ser. No. 917,794 entitled "Method and Apparatus for Monitoring and Controlling a Plurality of Air Guns for Seismic Surveying" describes a solenoid valve housing provided with a socket having a pressure sensor transducer mounted therein. A passage extending from the socket through the housing communicates with a passage leading into the air gun for transmitting to the transducer pressure variations occurring within the air gun when the gun is fired. The flat circular end face of the pressure-sensitive transducer is positioned transversely with respect to the axis of the passage leading from the air gun. An enormous pressure surge is applied against the flat end of the transducer when the air gun is fired.
Also, when the air gun is fired, there are large axial acceleration forces involved as the air gun recoils during the fast acceleration of the shuttle at the instant of firing. Accordingly, the transducer must be able to withstand tremendous acceleration forces when the air gun is fired. In order to handle such acceleration, the pressure-sensitive transducer is formed by multiple wafers of piezoelectric material with reverse polarities, thereby being non-responsive to axial acceleration. These wafer elements are electrically connected so that when actuated by pressure changes from the air gun, the voltages generated by the individual wafers are additive. The provision of multiple layers or wafers for the pressure-sensitive transducer are more difficult to fabricate, are subject to fracture and are less rugged than single crystal sensors. The present invention is directed to an improvement of the apparatus described in the aforesaid application.
As used herein, the term "gas" is used in a broad sense to include any compressible gaseous fluid which may be utilized in an air gun, and it includes (but is not limited to) air, steam, nitrogen, carbon dioxide, gaseous products of combustion, and mixtures thereof.